Some of his chief strengths — his facility with words, his wry detachment, his reasoning skills, his youthful cool — have not always served him well and may pose significant vulnerabilities in the series of presidential debatesthat begins Friday, according to political analysts and a review of his earlier debate performances.

Mr. Obama has a tendency to overintellectualize and to lecture, befitting his training as a lawyer and law professor. He exudes disdain for the quips and sound bites that some deride as trivializing political debates but that have become a central part of scoring them. He tends to the earnest and humorless when audiences seem to crave passion and personality. He frequently rises above the mire of political combat when the battle calls for engagement.

This was seen most starkly at last month’s forum at Saddleback Church, where he and Senator John McCainwere interviewed back to back by the evangelical church’s pastor, the Rev. Rick Warren. Mr. Obama gave long, discursive answers to questions on loaded topics like abortion and personal moral failings, while Mr. McCain stole the show with earthy anecdotes and humor.

“Obama clearly knows how to float like a butterfly,” said Alan Schroeder, who studies media and the presidency at Northeastern University, “but he needs to work on the sting-like-a-bee part.”

Those who watched his debate performances during the long primary season say he improved markedly from a fairly shaky start but never really mastered the form.

“During the course of 18 months, he learned to give shorter, crisper answers,” said Howard Wolfson, a top adviser to Senator Hillary Rodham Clintonwho helped prepare her for the two dozen debates she had with Mr. Obama and other Democratic candidates. “I think his command of facts, figures and anecdotes got consistently stronger.”

One of Mr. Obama’s worst moments came in the first Democratic debate, in South Carolina in April 2007. The candidates were asked how they would respond to a new series of terrorist attacks.

Mrs. Clinton gave a short answer, ending, “Let’s focus on those who have attacked us and do everything we can to destroy them.”

But Mr. Obama gave a rambling reply on emergency response, intelligence flaws and the importance of engaging “the international community.” He had to ask for a second chance to answer the question in order to say he would go after the terrorists.

Two months later he was on the defensive over a question of meeting without preconditions with the leaders of hostile states. He said that he would do so and that he disagreed with the Bush administration’s approach of not engaging with Iran, Syria, Cuba and North Korea. His rivals cited this as evidence of his naïveté in foreign affairs.

Perhaps Mr. Obama’s single worst debate moment came early this January in New Hampshire, where Mrs. Clinton was asked why some people found her less likable than some of her rivals. She adopted a hurt tone and said of Mr. Obama: “He’s very likable, I agree with that. But I don’t think I’m that bad.”

Mr. Obama looked at her and said coldly, “You’re likable enough.”

In part because of a sympathy backlash, Mrs. Clinton went on to win the New Hampshire primary, keeping her candidacy alive.

At a debate two weeks later, Mr. Obama apologized for the remark.

From then on, Mr. Obama’s performances improved. At a debate in late January, Mrs. Clinton accused him of getting financial favors from a Chicago “slumlord,” a reference to Antoin Rezko. Mr. Obama shot back that while he was doing community organizing work, she had been sitting on the board of Wal-Mart. But he seemed uncomfortable with the exchange and a few minutes later lightened a tense moment with his answer to a question about whether Bill Clintonhad been the first “black president.”

“I would have to investigate more Bill’s dancing abilities and some of this other stuff,” he said with a smile, “before I accurately judged whether in fact he was a brother.”

And, though he focused largely on his Democratic rivals during most of the primary campaign, he said of Mr. McCain at a Democratic debate in February, “Somewhere along the line the Straight Talk Express lost some wheels.”

Again in February, when Mrs. Clinton tried to goad him over his borrowing some speech lines, mockingly referring to “change you can Xerox,” he brushed off the swipe as part of the “silly season in politics.”

By the final debate of the primary season, on April 16 in Philadelphia, Mr. Obama was both more polished and at times exasperated by the process. He was subjected to repeated severe questioning that night about the Rev. Jeremiah A. Wright Jr., his association with William Ayers, a former member of the Weather Underground, and his refusal then to wear a flag pin. He endured the grilling, looking testy at times, but also won some points for empathy by recounting the stories of people he had met along the campaign trail, including a man in Latrobe, Pa., “who lost his job and was trying to figure out where he was going to get the gas money to go find a job.”

Mr. Obama has been involved in high-stakes political debates since his first, unsuccessful race for Congress in 2000, when he took on a popular incumbent, Representative Bobby L. Rush of Illinois.

In the sole televised debate of that campaign, he aggressively attacked Mr. Rush until the moderator cut him off. Mr. Rush dismissed his challenger as an upstart with no record to run on.

“Just what’s he done?” Mr. Rush said, according to an account in “Obama: From Promise to Power,” by David Mendell. “I mean, what’s he done?”

A deflated Mr. Obama went on to lose the election by 30 percentage points.

In 2004, Mr. Obama was running for the Senate against Alan Keyes, a glib talk show host whom Republicans had imported from Maryland to oppose him after more credible candidates were disqualified by personal scandal.

Mr. Obama’s aloofness and windiness were on display in a debate against Mr. Keyes, but he was able to cut through the verbal clutter on occasion to deliver a biting response to him.

“Your logic was not that complicated,” Mr. Obama said to end one exchange on homosexuality and incest. “It’s just wrong.”

On another occasion, Mr. Keyes asserted that Jesus would not vote for Mr. Obama because of his support for abortion rights. Mr. Obama replied that if he had a chance to consult with Jesus, he would not ask him about a Senate race.

“I’d want to know if I was going up or down,” he said. “There’s all sorts of questions that I’d be interested in.”